Porosity has long been recognized as an important detrimental factor affecting mechanical properties and performance of cast components. Shrinkage and gas precipitation are two main sources for porosity formation in aluminum castings. For a given casting process and a design of casting geometry and gating/riser system, alloy composition, in particular trace elements such as strontium (Sr), phosphorus (P), bismuth (Bi), calcium (Ca), and others, can play an important role in solidification characteristics and thus porosity formation.
It is well known that modification of the acicular silicon present in cast Al—Si alloys to a fine fibrous form results in an improved strength and ductility for these alloys. The technology of modification has matured since Pasz first introduced sodium (Na) to modify eutectic silicon in 1920. Additions made for modification and refinement of silicon structure now include strontium (Sr) and antimony (Sb). Several reports have appeared in the literature of negative interactions between antimony and either strontium or sodium. It was reported that Sb concentrations as low as 100 ppm can significantly affect the tensile elongation of Sr modified A356 alloy. This deterioration in mechanical properties was exacerbated by the presence of phosphorus (P). P. V. Bonsignore, E. J. Daniels and C. T. Wu, Calcium Metal as a Scavenger for Antimony from Aluminum Alloys, Argonne National Laboratory, Technical Report, Oct. 4, 1994. To achieve the similar eutectic silicon morphology in the presence of varying levels of P, sufficient Sr needs to be added. FIG. 1 quantitatively shows that higher Sr concentrations are required for retaining good modification when P neutralization of the Sr effects is considered. M. Garat and R. Scalliet, A review of recent French casting alloy development, AFS Transactions, vol. 86 (1978), pp 549-562.
Phosphorus is an impurity associated with silicon used in the alloy. The effect of P, at or beyond concentrations of a few of ppm, is not only to perform the function of nucleating primary Si in eutectic or hyper-eutectic but also to yield a distinctly acicular eutectic silicon structure in Al—Si hypoeutectic alloys. It was also found that both the number of primary cc-dendrites and the dendrite arm spacing (DAS) was increased in the high-purity Al-10% Si alloy by the addition of 0.005% (50 ppm) of phosphorus. C. R. Loper and J.-I. Cho, Influence of trace amounts of phosphorus in Al casting alloys—A review of the literature, vol. 108 (2000), pp. 667-672.
Magnesium also tends to coarsen the eutectic silicon structure and thus reinforces the effect of P. For example, an Al-7% Si alloy containing 2 ppm P still exhibits a lamellar silicon structure, whereas Al-7% Si-0.3% Mg alloy also containing 2 ppm P is acicular. M. Garat and R. Scalliet, A review of recent French casting alloy development, AFS Transactions, vol. 86 (1978), pp 549-562.
Like phosphorus, bismuth also neutralizes the effect of Sr modification. To retain full modification, Sr/Bi mass ratio higher than 0.45 is required when bismuth is present in the melt. S. Farahany, A. Ourdjini, M. H. Idris, L. T. Thai, Effect of bismuth on microstructure of unmodified and Sr-modified Al-7Si-0.4 Mg alloys, Trans. Nonferrous Met. Soc. China vol. 21 (2011), pp 1455-1464. S. Farahany, A. Ourdjini, M. H. Idrsi, S. G. Shabestari, Evaluation of the effect of Bi, Sb, Sr and cooling condition on eutectic phases in an Al—Si—Cu alloy (ADC12) by in situ thermal analysis, Thermochimica Acta, 559 (2013) 59-68. N. R. Rathod, J. V. Manghani, Effect of Modifier and Grain Refiner on Cast Al-7si Aluminum Alloy: A Review, International Journal of Emerging Trends in Engineering and Development, Issue 2, Vol. 5. (JULY-2012), pp. 574-582.
In spite of the positive effect of Sr modification on tensile strength and particularly ductility, excessive modification increases the tendency of microporosity due to the change of solidification characteristics and formation of dual primary and eutectic grain structures Q. G. Wang, D. Apelian, L. Arnberg, S. Gulbrandsen-Dahl, and J. Hjelen, Solidification of the Eutectic in Hypoeutectic Al—Si Alloys, AFS Transactions, vol. 107 (1999), pp. 249-256. It has also been reported that the excessive eutectic modification delays the formation of an impermeable casting skin and thus increases core gas penetration from the sand cores, resulting in gas bubbles in the solidified castings. (These exogenous gas bubbles are distinct from gas porosity resulting from the rejection of hydrogen dissolved in the liquid aluminum during solidification.)
Therefore, it is important in aluminum casting to properly control eutectic modification levels to minimize macro shrinkage porosity and gas bubbles simultaneously. The disclosed methods, systems, and articles of manufacture in this invention are intended to solve this problem.